Multi-value amplitude modulation in optical communication is a modulation method for modulating intensity of light by three or more levels in order to transmit information of a plurality of bits with one symbol. According to multi-value amplitude modulation, optical transmission can be sped up (for example, refer to U.S. Patent Publication No. 2015/0071651 and Japanese Laid-open Patent Publication No. 2016-139976). Multi-value amplitude modulation in optical communication is carried out by any of direct modulation of a semiconductor laser, external modulation by an electro-absorption (hereafter, referred to as EA) modulator, and external modulation by a Mach-Zehnder (hereafter, referred to as MZ) modulator (for example, refer to Japanese Laid-open Patent Publication No. 2016-139976).
A technique for applying pre-equalization to multi-value amplitude modulation by an MZ modulator has been reported (for example, refer to Japanese Laid-open Patent Publication No. 2016-139976). The report describes that pre-equalization is not applied to external modulation by an EA modulator and direct modulation of a semiconductor laser. In this case, pre-equalization refers to a technique for adjusting an electrical signal that is applied to a MZ optical modulator and suppressing waveform deformation of an optical signal thereby, where the waveform deformation is due to a group delay time difference.